1. Field of the Invention
The present invention relates to an electric motor which has an air purge function for preventing the entry of foreign matter to the inside of the electric motor.
2. Description of the Related Art
An electric motor which drives a machine tool is sometimes used in an environment where cleaning fluid, cutting fluid, etc. are sprayed. To prevent cleaning fluid, cutting fluid, or other foreign matter from entering inside the electric motor, it is known to provide a seal member at joined parts of members which form the housing or to provide an oil seal between the housing and shaft. However, for example, in the case of an electric motor which operates at a high speed such as over 8000 rpm, the oil seal may become worn and cannot provide the desired sealing effect.
Known in the art is an electric motor which has an air purge function for preventing the entry of foreign matter to the inside of the electric motor without using an oil seal. For example, known in the art is a spindle device of a machine tool which is designed to introduce compressed air to the inside of an electric motor through a tube which is attached to an input shaft side and to discharge compressed air through the air gap between the stator core and rotor from the opposite output shaft side to the outside of the electric motor (see JP2007-105850A).
Further, known in the art is an electric motor which is provided with an air purge device attached to an end face at an output shaft side and having an air flow path which extends straight toward the inside in the radial direction (see JP2013-236473A).
However, when feeding compressed air from the opposite input shaft side to the output shaft side, the flow path of the compressed air tends to be long, and therefore the pressure loss increases. Further, since the air gap between the stator core and the rotor is generally small, the pressure loss may be even greater. For this reason, the efficiency of utilization of air tends to be low and a large amount of air has to be fed to provide the air purge function. Further, since a bent air flow path has to be formed, the structure becomes complicated and the production cost increases.
In the case where an air feed port is provided at the output shaft side and compressed air is fed through a straight flow path as disclosed in JP2013-236473A, the rate of feed of air becomes excessive at the time of acceleration or at the time of steady operation of the electric motor and the efficiency drops. Further, when the supply of air becomes unstable and the amount of air temporarily becomes insufficient, entry of foreign matter to a portion near the output shaft may not be able to be prevented.
Therefore, there is a need for an electric motor which has an air purge function with improved efficiency of utilization of air.